function onChange(){
  var tprice, quantity, eprice;
  quantity = $('quantity').value;
  eprice = $('product_baglist').value;
  tprice = eprice * quantity;
  alert(tprice);
} //end of onChange


document.observe('dom:loaded', function() {
	$('product_baglist').observe('change', onChange);
    $('quantity').observe('change', onChange);

});

